With today's portable communication products becoming smaller and more compact, the designs for these products, from both an electrical and mechanical perspective, are moving towards miniaturization. The miniaturization of both electrical and mechanical components to accommodate limited space constraints has lead to an increase in issues relating to radio performance.
An example of the space constraint issue associated with miniaturization becomes apparent in voltage controlled oscillator (VCO) technology. Voltage controlled oscillator circuits provide the operating frequencies with which communications products transmit or receive signals. In portable radio applications, a voltage controlled oscillator usually employs a frequency tuning element to tune its operating frequency. The oscillator is typically shielded using a metalized can to maintain the stability of the tuned frequency as well as to protect the VCO from picking up noise which can degrade the radio performance. The noise performance specifications associated with a VCO module are critical to the performance of a radio. Frequency trimming is necessary to obtain the highest performing phase noise oscillators. By trimming the VCO frequency, the proper centering of the VCO frequency can be accomplished. Unfortunately, the metalized can itself creates design problems related to the frequency tuning of the VCO which must be addressed by circuit designers.
As is well known, the narrower the bandwidth of the oscillator, the better the noise performance. A disadvantage associated with today's VCO modules is that the metalized can shifts the center frequency of the VCO. This forces designers to design the VCO to operate at a different center frequency taking into account the amount of shift that will occur when the can is placed over the oscillator circuit. In order to accommodate for error margins, designers must provide extra bandwidth in the VCO design. However, increasing the bandwidth of the VCO degrades noise performance.
Another way circuit designers address the issue of frequency shift in VCO modules is to include a window or hole in the metalized can through which the tuning element can be accessed. However, the smaller size constraints of today's portable communications products make this option less feasible. The window must be large enough to access the component which, if left uncovered, can negate the effects of the shield. Placing a cover over the window after tuning the VCO requires an extra step in the processing of the module and increases the parts count.
Accordingly, there is a need for an improved voltage controlled oscillator module which eases the tuning of such devices.